The present invention relates to semiconductor devices, and more specifically, to fin-type field effect transistors (finFETs).
Variable reactors, also known as variable capacitors and “varactors” are electronic devices (e.g., solid-state diodes) that have a capacitance that is controlled by a suitable voltage or current bias. For example, as a reverse voltage is applied to a PN junction of a varactor diode, electrons in the n-region are attracted to the cathode while holes in the p-region are attracted to the anode. This results in the formation of a depletion region between the anode and cathode that behaves as the dielectric of the device. As the applied reverse voltage increases, the depletion region (i.e., dielectric) widens, while the capacitance across the PN junction decreases since capacitance is inversely proportional to dielectric thickness. Therefore, by varying the reverse voltage across a PN junction of a varactor diode, the junction capacitance can be varied.
The variability of the capacitance in a varactor may be employed to provide useful function in various types of electronic circuits such as, for example, amplifiers, oscillators, and frequency synthesizers. As a non-limiting example, varactors may be used to construct voltage-controlled oscillators (VCOs), which can generate a tunable stable frequency (e.g., controllable by an applied current or voltage) without employing a circuit with multiple oscillators. A VCO is a versatile basic building block for constructing transceiver circuitry, phase locked loop (PLL) circuitry and other wireless communication circuitry. In such instances, VCOs may be used when a variable frequency is required or when a signal is to be synchronized to a reference signal, as non-limiting examples. Varactors are also used in other electronic application including, for example, frequency multipliers, harmonic generators, and modulation/demodulation operations.
In general, varactor designs take into account a number of design parameters. One such parameter is “tunability,” which is typically defined as the ratio between the highest and lowest capacitive values (Cmax/Cmin) over the range of applied voltages for the circuit. Another property is “linearity.” There are two definitions for “linearity”: (i) 1/√{square root over (C)}; and (ii)
            ⅆ              (                  ln          ⁢                                          ⁢          C                )                    ⅆ      V        ,where C is the voltage-dependent varactor capacitance. In the first case (i), it is desirable that 1/√{square root over (C)} be a straight line and in the second case (ii) that
      ⅆ          (              ln        ⁢                                  ⁢        C            )            ⅆ    V  be a constant, both as V varies. Yet another design parameter that is taken into account the quality factor “Q”, which is a function of the series resistance of the diode and the capacitive value of the varactor at the higher frequency ranges of the circuit.